i gues since shizuka is the one who made zero a vampire..
zero needed shizuka's blood..
and since kaname drank shizuka's blood.. and shizuka's already dead..
kaname allowed zero to drink shizuka's blood from his blood.

1) Zero can never be a vampire 'noble' at all, since he was human when bitten. Vampire nobles are the next thing to pureblood; they come from long vampiric lineages with only small amounts of human blood infiltrating their family trees.

2) "He needed blood from an almighty pure-blood"

It isn't enough to drink the blood of ANY pureblood. You need the blood of the one who turned you into a vampire; in this case, Shizuka. That was why Kaname mentioned that he left some of Shizuka's blood behind in her body for Zero to drink (though Ichiru ended up drinking it), and the leader of the Hunters' Association said drinking Kaname's blood would only temporarily stave off Zero's fall to E-class.

This exchange of blood is a common factor in most vampire media and literature, and VK is no exception. Note the way Kaname turned Yuuki into a vampire...he bit her, and then gave her his blood.

3) When Zero bit Yuuki, her vampire side was 'sealed away', so her blood probably had all the qualities of a normal human. Now that she's 'awakened', her blood inside Zero might certainly have some interesting effects...but back at the time you refer to, it didn't.

Also, note that at the time Zero was advised to bite Kaname, the person advising him--indeed, most persons---didn't know that Yuuki was a pureblood, or they would simply have told him to bite her instead.

I agree with Tamasha. However, when the leader of the Hunters' Association said drinking Kaname's blood would only temporarily stave off Zero's fall to E-class, I'm not sure she knew that Kaname was the one who killed Shizuka. I also have some suspicions about this supposed leader of the Hunters' Association. With that said, it may be possible that Zero won't fall to E-class because he drank Kaname's blood and indirectly received some of Shizuka's blood. I know Kaname didn't relish the idea of having Zero feed from him, but may have wanted 1) for Zero to have the benefit of Shizuka's blood 2) for Zero to no longer feed off of Yuuki, and 3) gain enough power to protect Yuuki and defeat Rido. There's a lot of loathing going on between Kaname and Zero.

w8'3x....... wat about dis part *Yuuki is revealed to be a pureblood vampire of the Kuran line* and also this *She's also very sensitive to sunlight, as it was the first time she ever saw sunlight with her vampire eyes. Before her memory loss, her parents kept her confined in a well-furnished undeground chamber, preventing other vampires from finding out about her.* and this *Yuuki's parents, raised Kaname as Yuuki's brother, although Kaname never fully became 'their son'. Yuuki and Kaname planned to marry as children, following the example of Juuri and Haruka who were also brother and sister and married.*
sorry if its random ^^

Kaname planned it all along. He said that the Chess piece he was raising is Zero. A vampire Hunter, A vampire that has taken the blood of 2 purebloods, Yuuki and Kaname. Also Kaname took the powers of Shizuka so it makes 3. And as of now, Zero is the only one who could lift the curse that Rido casted on Kaname since he has enough power to do this.. I'm also confused about this.

W8. Aidou-sempai also drank yuuki's blood! [the wound in her hand from falling a tree.] So, Aidou-sempai would get stronger...hmmm...but i think the amount of blood is pretty small. But I also think the blood of a pureblood is very powerful even with a single drop of it. hmmm.....so confusing. really.

Kaname planned it all along. He said that the Chess piece he was raising is Zero. A vampire Hunter, A vampire that has taken the blood of 2 purebloods, Yuuki and Kaname. Also Kaname took the powers of Shizuka so it makes 3. And as of now, Zero is the only one who could lift the curse that Rido casted on Kaname since he has enough power to do this.. I'm also confused about this.

W8. Aidou-sempai also drank yuuki's blood! [the wound in her hand from falling a tree.] So, Aidou-sempai would get stronger...hmmm...but i think the amount of blood is pretty small. But I also think the blood of a pureblood is very powerful even with a single drop of it. hmmm.....so confusing. really.

Yuuki's vampire side was still inactive back then so........ Aidou is still Aidou ^^

well... the reason to it was because... when Zero bit Yuuki... her vampire said was still not awaken... it was enclosed... thus making her a pure normal human... she didn't even knew she was a vampire so....

I don't know if this has been said but the Vampire Hunters were rumored to have eaten a vampire to gain the power to fight agaisnt vampires. So not only does Zero have this Vampire Hunter powers but he has also taken the blood of Yuuki and Kaname. Kaname who has feasted on the blood of Shizuka gave Zero his blood. In fact

Spoiler Alert! Click to show or hide

when Kaname went to see Zero when Zero was in the jail in the last manga (39). He told Zero that by taking Yuuki's blood all those times he helped to awaken her and now that Yuuki has awoken the power that was in her blood is now awakening in Zero. Kaname says that "The original power of the Kuran must have already started awakening in your body.

So not only does Zero have his original Vampire Hunter powers but he has taken the blood of 3 purebloods, Kaname's, Yuuki's and Shizuka. And then there is that myth about Vampire Hunter twins. It's enough to make your head spin.
This was all done so that Zero will become the "king" that will soon eat everyone. Kaname knew exactly what he was doing.